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  • 1
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    Sensitivity of Axisymmetric Tropical Cyclone Spinup Time to Dry Air Aloft (2016)
    American Meteorological Society
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    Publikationsdatum: 2016-10-06
    Beschreibung: The sensitivity of tropical cyclone spinup time to the initial entropy deficit of the troposphere is examined in an axisymmetric hurricane model. Larger initial entropy deficits correspond to less moisture above the initial lifting condensation level of a subcloud-layer parcel. The spinup time is quantified in terms of thresholds of integrated horizontal kinetic energy within a radius of 300 km and below a height of 1.5 km. The spinup time increases sublinearly with increasing entropy deficit, indicating the greatest sensitivity lies with initial moisture profiles closer to saturation. As the moisture profile approaches saturation, there is a large increase in the low-level, area-averaged, vertical mass flux over the spinup period because of the predominance of deep convection. Higher entropy deficit experiments have a greater amount of cumulus congestus and reduced vertical mass flux over a longer duration. Consequently, the secondary circulation takes longer to build upward, and the radial influx of angular momentum is reduced. There is also a reduction in the conversion of potential available enthalpy to horizontal kinetic energy, as a result of reduced flow down the radial pressure gradient early in the spinup period. Later in the spinup period, the low-level vortex spins up relatively quickly near the nascent radius of maximum wind in the high-entropy deficit experiments, whereas the low-level vortex spins up over a wider area in the low-entropy deficit experiments.
    Print ISSN: 0022-4928
    Digitale ISSN: 1520-0469
    Thema: Geographie , Geologie und Paläontologie , Physik
    Publiziert von American Meteorological Society
    Standort Signatur Erwartet Verfügbarkeit
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  • 2
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    An Ensemble Approach to Investigate Tropical Cyclone Intensification in Sheared Environments. Part I: Katia (2011) (2015)
    American Meteorological Society
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    Publikationsdatum: 2015-12-11
    Beschreibung: The mechanisms responsible for tropical cyclone (TC) intensification in the presence of moderate vertical shear magnitudes are not well understood. To investigate how TCs intensify in spite of moderate shear, this study employed a 96-member ensemble generated with the Advanced Hurricane Weather Research and Forecasting (AHW) Model. In this first part, AHW ensemble forecasts for TC Katia (2011) were evaluated when Katia was a weak tropical storm in an environment of 12 m s−1 easterly shear. The 5-day AHW forecasts for Katia were characterized by large variability in the intensity, presenting an opportunity to compare the underlying mechanisms between two subsets of members that predicted different intensity scenarios: intensification and weakening. The key difference between these two subsets was found in the lower-tropospheric moisture north of Katia (i.e., right-of-shear quadrant). With more water vapor in the lower troposphere, buoyant updrafts helped to moisten the midtroposphere and enhanced the likelihood of deep and organized convection in the subset that predicted intensification. This finding was validated with a vorticity budget, which showed that deep cyclonic vortex stretching and tilting contributed to spinning up the circulation after the midtroposphere had moistened. Sensitivity experiments, in which the initial conditions were perturbed, also demonstrated the importance of lower-tropospheric moisture, which suggests that moisture observations may help reduce uncertainty in forecasts of weak, sheared tropical storms.
    Print ISSN: 0022-4928
    Digitale ISSN: 1520-0469
    Thema: Geographie , Geologie und Paläontologie , Physik
    Publiziert von American Meteorological Society
    Standort Signatur Erwartet Verfügbarkeit
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  • 3
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    An Ensemble Approach to Investigate Tropical Cyclone Intensification in Sheared Environments. Part II: Ophelia (2011) (2016)
    American Meteorological Society
    Details
    Publikationsdatum: 2016-02-26
    Beschreibung: The mechanisms leading to tropical cyclone (TC) intensification amid moderate vertical wind shear can vary from case to case, depending on the vortex structure and the large-scale conditions. To search for similarities between cases, this second part investigates the rapid intensification of Hurricane Ophelia (2011) in an environment characterized by 200–850-hPa westerly shear exceeding 8 m s−1. Similar to Part I, a 96-member ensemble was employed to compare a subset of members that predicted Ophelia would intensify with another subset that predicted Ophelia would weaken. This comparison revealed that the intensification of Ophelia was aided by enhanced convection and midtropospheric moisture in the downshear and left-of-shear quadrants. Enhanced left-of-shear convection was key to the establishment of an anticyclonic divergent outflow that forced a nearby upper-tropospheric trough to wrap around Ophelia. A vorticity budget showed that deep convection also contributed to the enhancement of vorticity within the inner core of Ophelia via vortex stretching and tilting of horizontal vorticity enhanced by the upper-tropospheric trough. These results suggest that TC intensity changes in sheared environments and in the presence of upper-tropospheric troughs highly depend on the interaction between convective-scale processes and the large-scale flow. Given the similarities between Part I and this part, the results suggest that observations from the three-dimensional moisture and wind fields could improve both forecasting and understanding of TC intensification in moderately sheared environments.
    Print ISSN: 0022-4928
    Digitale ISSN: 1520-0469
    Thema: Geographie , Geologie und Paläontologie , Physik
    Publiziert von American Meteorological Society
    Standort Signatur Erwartet Verfügbarkeit
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  • 4
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    Tropospheric and Stratospheric Precursors to the January 2013 Sudden Stratospheric Warming (2016)
    American Meteorological Society
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    Publikationsdatum: 2016-03-23
    Beschreibung: This paper investigates the tropospheric and stratospheric precursors to a major sudden stratospheric warming (SSW) that began on 6 January 2013. Using the Climate Forecast System Reanalysis dataset, the analysis identified two distinct decelerations of the 10-hPa zonal mean zonal wind at 65°N in December in addition to the major SSW, which occurred on 6 January 2013 when the 10-hPa zonal mean zonal wind at 65°N reversed from westerly to easterly. The analysis shows that the two precursor events preconditioned the stratosphere for the SSW. Analysis of the tropospheric state in the days leading to the precursor events and the major SSW suggests that high-latitude tropospheric blocks occurred in the days prior to the two December deceleration events, but not in the days prior to the SSW. A detailed wave activity flux (WAF) analysis suggests that the tropospheric blocking prior to the two December deceleration events contributed to an anomalously positive 40-day-average 100-hPa zonal mean meridional eddy heat flux prior to the SSW. Analysis of the stratospheric structure in the days prior to the SSW reveals that the SSW was associated with enhanced WAF in the upper stratosphere, planetary wave breaking, and an upper-stratospheric/lower-mesospheric disturbance. These results suggest that preconditioning of the stratosphere occurred as a result of WAF initiated by tropospheric blocking associated with the two December deceleration events. The two December deceleration events occurred in the 40 days prior to the SSW and led to the amplification of wave activity in the upper stratosphere and wave resonance that caused the January 2013 SSW.
    Print ISSN: 0027-0644
    Digitale ISSN: 1520-0493
    Thema: Geographie , Geologie und Paläontologie , Physik
    Publiziert von American Meteorological Society
    Standort Signatur Erwartet Verfügbarkeit
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  • 5
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    Factors Leading to Extreme Precipitation on Dominica from Tropical Storm Erika (2015) (2018)
    American Meteorological Society
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    Publikationsdatum: 2018-02-01
    Beschreibung: Tropical cyclones are generally characterized by strong rotating winds, and yet, the associated rainfall can be equally destructive. Tropical Storm Erika (2015) is an example of such a cyclone whose heavy rainfall south of the storm center was responsible for significant loss of life and property. Tropical Storm Erika was a weak tropical storm in a sheared environment that passed through the Lesser Antilles on 27 August 2015. Radar and rain gauges measured at least a half meter of rainfall on the Commonwealth of Dominica in about 5 h. In this study, an analysis of several observational datasets showed that the combination of a sheared environment, dry northern sector, and mesovortex contributed to the significant storm precipitation. The sheared environment affected the storm structure, causing it to weaken, but also organized convection and precipitation in the region that passed over Dominica. Furthermore, a mesovortex embedded within the storm persisted over Dominica, leading to enhanced rainfall totals. Understanding the factors leading to heavy rainfall for this case is important for future prediction of similar weak, sheared tropical storms passing near mountainous islands.
    Print ISSN: 0027-0644
    Digitale ISSN: 1520-0493
    Thema: Geographie , Geologie und Paläontologie , Physik
    Publiziert von American Meteorological Society
    Standort Signatur Erwartet Verfügbarkeit
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  • 6
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    Climatological Analysis of Tropical Cyclone Intensity Changes under Moderate Vertical Wind Shear (2017)
    American Meteorological Society
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    Publikationsdatum: 2017-04-17
    Beschreibung: Although infrequent, tropical cyclones (TCs) can intensify under moderate vertical wind shear (VWS). A potential hypothesis is that other factors—associated with both the TC and its environment—can help offset the effects of VWS and aid intensification. This hypothesis was tested with a large dataset of 6-hourly best tracks and environmental diagnostics for global TCs between 1982 and 2014. Moderate VWS was objectively defined as 4.5–11.0 m s−1, which represents the 25th–75th percentiles of the global distribution of 200–850-hPa VWS magnitude around TCs. Intensifying events (i.e., unique 6-hourly data points) were compared against steady-state events to determine which TC and environmental characteristics favored intensification under moderate VWS. This comparison showed that intensifying events were significantly stronger, closer to the equator, larger, and moving with a more westward motion than steady-state events. Furthermore, intensifying events moved within environments characterized by warmer sea surface temperatures, greater midtropospheric water vapor, and more easterly VWS than steady-state events. Storm-relative, shear-relative composites suggested that the coupling between water vapor, surface latent heat fluxes, and storm-relative flow asymmetries was conducive for less dry air intrusions and more symmetric rainfall in intensifying events. Last, the comparison showed no systematic differences between environmental wind profiles possibly due to the large temporal variability of VWS.
    Print ISSN: 0027-0644
    Digitale ISSN: 1520-0493
    Thema: Geographie , Geologie und Paläontologie , Physik
    Publiziert von American Meteorological Society
    Standort Signatur Erwartet Verfügbarkeit
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  • 7
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    A Hypothesis for the Intensification of Tropical Cyclones under Moderate Vertical Wind Shear (2018)
    American Meteorological Society
    Details
    Publikationsdatum: 2018-11-14
    Beschreibung: A major open issue in tropical meteorology is how and why some tropical cyclones intensify under moderate vertical wind shear. This study tackles that issue by diagnosing physical processes of tropical cyclone intensification in a moderately sheared environment using a 20-member ensemble of idealized simulations. Consistent with previous studies, the ensemble shows that the onset of intensification largely depends on the timing of vortex tilt reduction and symmetrization of precipitation. A new contribution of this work is a process-based analysis following a shear-induced midtropospheric vortex with its associated precipitation. This analysis shows that tilt reduction and symmetrization precede intensification because those processes are associated with a substantial increase in near-surface vertical mass fluxes and equivalent potential temperature. A vorticity budget demonstrates that the increased near-surface vertical mass fluxes aid intensification via near-surface stretching of absolute vorticity and free-tropospheric tilting of horizontal vorticity. Importantly, tilt reduction happens because of a vortex merger process—not because of advective vortex alignment—that yields a single closed circulation over a deep layer. Vortex merger only happens after the midtropospheric vortex reaches upshear left, where the flow configuration favors near-surface vortex stretching, deep updrafts, and a substantial reduction of low-entropy fluxes. These results lead to the hypothesis that intensification under moderate shear happens if and when a “restructuring” process is completed, after which a closed circulation favors persistent vorticity spinup and recirculating warm, moist air parcels.
    Print ISSN: 0022-4928
    Digitale ISSN: 1520-0469
    Thema: Geographie , Geologie und Paläontologie , Physik
    Publiziert von American Meteorological Society
    Standort Signatur Erwartet Verfügbarkeit
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  • 8
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    Clouds and Convective Self‐Aggregation in a Multimodel Ensemble of Radiative‐Convective Equilibrium Simulations (2020)
    American Geophysical Union
    Details
    Publikationsdatum: 2020-09-01
    Digitale ISSN: 1942-2466
    Thema: Geographie , Geologie und Paläontologie
    Publiziert von American Geophysical Union
    Standort Signatur Erwartet Verfügbarkeit
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  • 9
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    Adopting Model Uncertainties for Tropical Cyclone Intensity Prediction (2014)
    American Meteorological Society
    Details
    Publikationsdatum: 2014-01-01
    Beschreibung: Quantifying and reducing the uncertainty of model parameterizations using observations is evaluated for tropical cyclone (TC) intensity prediction. This is accomplished using a nonlinear inverse modeling technique that produces a joint probability density function (PDF) for a set of parameters. The dependence of estimated parameter values and associated uncertainty on two types of observable quantities is analyzed using an axisymmetric hurricane model. When the observation is only the maximum tangential wind speed, the joint PDF of parameter estimates has large variance and is multimodal. When the full kinematic field within the inner core of the TC is used for the observations, however, the joint parameter estimates are well constrained. These results suggest that model parameterizations may not be optimized using the maximum wind speed. Instead, the optimization should be based on observations of the TC structure to improve the intensity forecasts.
    Print ISSN: 0027-0644
    Digitale ISSN: 1520-0493
    Thema: Geographie , Geologie und Paläontologie , Physik
    Publiziert von American Meteorological Society
    Standort Signatur Erwartet Verfügbarkeit
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  • 10
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    Impacts of Radiation and Cold Pools on the Intensity and Vortex Tilt of Weak Tropical Cyclones Interacting with Vertical Wind Shear (2020)
    American Meteorological Society
    Details
    Publikationsdatum: 2020-02-01
    Beschreibung: Idealized numerical simulations of weak tropical cyclones (e.g., tropical depressions and tropical storms) in sheared environments indicate that vortex tilt reduction and convective symmetrization are key structural changes that can precede intensification. Through a series of ensembles of idealized numerical simulations, this study demonstrates that including radiation in the simulations affects the timing and variability of those structural changes. The underlying reason for those effects is a background thermodynamic profile with reduced energy available to fuel strong downdrafts; such a profile leads to weaker lower-tropospheric ventilation, greater azimuthal coverage of clouds and precipitation, and smaller vortex tilt with radiation. Consequently, the simulations with radiation allow for earlier intensification at stronger shear magnitudes than without radiation. An unexpected finding from this work is a reduction of both vortex tilt and intensity variability with radiation in environments with 5 m s−1 deep-layer shear. This reduction stems from reduced variability in nonlinear feedbacks between lower-tropospheric ventilation, cold pools, convection, and vortex tilt. Sensitivity experiments confirm the relationship between those processes and suggest that microphysical processes (e.g., rain evaporation) are major sources of uncertainty in the representation of weak, sheared tropical cyclones in numerical weather prediction models.
    Print ISSN: 0022-4928
    Digitale ISSN: 1520-0469
    Thema: Geographie , Geologie und Paläontologie , Physik
    Publiziert von American Meteorological Society
    Standort Signatur Erwartet Verfügbarkeit
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